The portable telephone system is not limited to speech communication, but service capable of transmitting mail using characters by especially making a contract has been heretofore spread under, for example, the name of “short mail.” As a result, it is possible to conduct mail transmission similar to E mail using a personal computer or the like, from a mobile terminal such as a portable telephone. As a system of portable telephone of the next generation, which can be used in a plurality of countries, replacing the existing communication system which takes a country as the unit, systems according to IMT (International Mobile Telecommunication) 2000 (or W-CDMA) and CDMA (Code Division Multiple Access) are about to be put to practical use. In these systems, transmission of mail called “short message” becomes possible.
As for the short message, a mobile terminal device of a transmission source (hereafter it is supposed that a mobile telephone is used) sends a created message and data of a transmission destination to a message center, and the message center transmits the message to a portable telephone of the transmission destination. Even in the case where the message is not delivered to the opposite party, the message center, for example, preserves the message in a server for several days, and transmits the message automatically or in response to a transmission request issued by the user. After the message center has transmitted the message, it erases the already transmitted message. The message center preserves a message that is not transmitted yet for a predetermined number of days, i.e., the term of validity.
Even if a message is transmitted from the message center in such a system, the message is not received normally by a portable telephone of transmission destination in some cases according to the state of the portable telephone of the transmission destination (such as, for example, a state in which power is turned off and the portable telephone is left as it is, or a state in which the user stays in an area where communication is impossible). Heretofore, therefore, proposals for ensuring delivery have been effected.
For example, in Japanese Patent Application Laid-Open No. 2000-201223, a radio wave reception level is measured for transmission and mail transmission is conducted only in the state in which transmission is possible. When communication is impossible, transmission mail is preserved. The radio wave reception level is monitored. Only when the reception level has improved, reception is conducted. Thereby, the success rate of mail reception is raised. In Japanese Patent Application Laid-Open No. 2000-349928, there is provided, besides setting means for setting transmission (information to be transmitted, transmission time, and transmission destination), a transmission information storage memory for storing the transmission information when the transmission information cannot be transmitted. When the transmission time corresponding to the contents stored in the memory is reached, the information is transmitted to the pertinent transmission destination. Retransmission can be thus conducted. Furthermore, in Japanese Patent Application Laid-Open No. 2000-270039, a check is effected to determine whether there is an error in received data when a terminal device receives a message. When there is an error, data retransmission is requested. It is thus ensured that a correct message will be transmitted to the terminal device side certainly.
However, the conventional message delivery system cannot cope with situations where the mobile terminal device of transmission destination is not in the operation state or the reception is impossible. For example, when the mobile telephone is left as it is with power in the off-state or the mobile telephone stays in an area where communication is impossible for many hours, the mobile telephone of the transmission destination cannot receive a short message from the message center. If the validity term of mail storage in the message center has expired in this state, then the message becomes invalid and the message is discarded in the message center and the message is not sent to the transmission destination. As a result, there occur situations where the message is not actually delivered although the message is expected on the transmission source side to be delivered. If the message to be transmitted contains important contents, there might occur a situation where a business chance is missed.